Drops column from existing table.
Drops (removes) column from existing table. Before the operation can proceed it commits current transaction. Hence transaction is committed regardless of the success or failure of the drop.
Dropping column does not lock the table for reading and also does not wait on any reads to finish. Drop column will also attempt to remove column files from all partitions, thus freeing up disk space right away. On Windows OS that may not be possible and file remove operation is postponed until files are released by all threads. However logical drop column will succeed on Windows in presence of active readers.
The following example deletes the column called
comment from the table
ALTER TABLE ratings DROP COLUMN comment